Transcript Pulp Quality Pulp Strength Delivery

Pulp Quality
Pulp Strength Delivery –
Martin MacLeod
Qualities of Kraft Pulp
•Strong – Kraft means “strength”
•Can pulp any wood type
•Sulfite pulp is sensitive to extractives
•Good recovery system
•Hard to bleach
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Pulp Quality
Pulp Strength Delivery
Delivering strong pulp is a goal in
every mill but the commercial
digester appears to be our worst
enemy
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How is a mill pulp different than pulp
made in a pilot reactor?
There can be several differences
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How is a mill pulp different than pulp
made in a pilot reactor?
Most importantly – strength of commercial pulp is
always lower
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How is a mill pulp different than pulp
made in a pilot reactor?
Case A shows considerable strength loss –
Spruce/fir pulp of Kappa 31
Case B shows much less strength loss is
characteristic of Kamyr continuous digesters (but
not all Kamyrs) – Western hemlock Most
importantly – strength of commercial pulp is
always lower
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How is a mill pulp different than pulp
made in a pilot reactor?
All Types of Commercial Digesters Produce Weaker Pulp
Solid lines refer to pilot-plant pulps, dashed lines to commercial
B = Batch, C = Continuous
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Do Hardwoods Have This Problem?
NO
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Do Hardwoods Have This Problem?
NO – Shorter and more flexible
hardwood fibers appear to be able
to escape the digester without
damage
Softwood
Hardwood
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Explicit definition of strength delivery
Tear index of mill made pulp relative to its pilot-plant
reference pulp at a constant (mid-range) tensile strength
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Where in the fiber line is the strength
loss?
The digester appears to be the culprit
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Where in the fiber line is the strength
loss?
The digester appears to be the culprit –
The digester is where we get
significant changes in chemistry and
severe mechanical action on the fibers
during the blow
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How many mills have this problem?
MacLeod and others have looked at lots of
mill with regard to strength loss.
•All kraft mills pulping softwood show some
strength loss
•A bisulfite mill producing bleachable
grade softwood pulp did not yield any
strength loss
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How many mills have this problem?
The mills average about 75% performance but none come near
perfection.
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Where in the digester is the problem?
Do conditions in the digester lead to weaker pulp?
Is it the blow that weakens the pulp?
Is there something else going on here?
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Where in the digester is the problem?
To investigate strength loss in batch digesters, baskets were
hung in the digester to perform in-situ pilot cooks
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Where in the digester is the problem?
Pulp Sample
Strength Delivery
(%)-Basket in
upper position
Strength Delivery
(%)-Basket in
lower position
Pilot Plant
100
100
Basket
98
87
Washer
68
69
Blow line
58
59
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Where in the digester is the problem?
Lessons learned
•The pulp from the basket was somewhat weaker but not
the major source of strength loss
•Most (about 2/3) of the strength loss appears to occur
across the blow
•Blow line sampling can be trouble and should probably be
avoided
We might expect the same thing from a continuous digester
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but no studies have been done – at least that I know of.
A Happy Accident
A basket broke in one experiment causing the pulp to
be cooked twice
•Kappa 21; Viscosity of 21mPa.s
•Compared over cooked with blown pulp
•Pilot and mill cooked
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A Happy Accident
Effect of overcooking much less than that of damage in blow
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What is happening to the fibers?
A - Pilot made fibers have mild cell-wall damage
B – Blown mill fibers show severe damage and
frequent collapse of the cell wall
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How do you minimize the damage?
Displacement batch digesters (RDH,
SuperBatch) leave pulp cool and low
consistency at end of the cook
•Ideal for pumping out digester
•Applied in Sunds SuperBatch systems
•Potentially can be applied to conventional
digesters if digester can be cooled
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How well did pumping work?
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How well did pumping work?
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